U.S. Pat. No. 3,600,204 discloses a field of Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 glass frits that were developed for shaping and firing into crystalline ceramic (or glass-ceramic) articles having very low average thermal expansions by virtue of the beta-spodumene solid solution crystals therein and having excellent dimensional stability for long periods of time at temperatures up to 900.degree.C. One such frit has been particularly and advantageously employed in the manufacture of the several parts of composite regenerative heat exchanger bodies as described in U.S. Pat. Nos. 3,112,184 and 3,251,403.
U.S. Pat. No. 3,839,001 discloses an improvement in processing the glass frits yielding the above beta-spodumene glass-ceramics wherein a small quantity of a particulate crystalline seeding material is added to the frit prior to shaping and results in sintered bodies with more uniform thermal expansion hysteresis than the bodies without the seeding material additive. Both the base compositions and the improvement compositions are useful over the compositional range (expressed by weight as oxides) of 3.5-7.5% Li.sub.2 O, 15-30% Al.sub.2 O.sub.3 and 65-80% SiO.sub.2 with the mole ratio of Al.sub.2 O.sub.3 :Li.sub.2 O being between about 1.0-1.5. One of the seeding materials (although not the preferred material) is titania. Other seeding materials are beta-spodumene, beta-spodumene solid solution, alpha-spodumene, petalite, zirconia, zircon, silicon, and mixtures thereof.
Regenerators made using compositions in the ranges specified above have generally been suitable. However, although the expansions have been low, it would be desirable if the expansion over a normal operating temperature range of a gas turbine engine, i.e., about 25.degree.-800.degree.C., be substantially zero. Work toward such a goal taught that the silica level must be raised to the upper levels in the range stated to produce zero expansion bodies up to 800.degree.C., but that at the higher silica levels (greater than 74% silica) crystallization of cristobalite became a problem, the cristobalite causing higher and more variable expansions in the crystallized bodies. And although the cristobalite could be avoided by firing to higher temperatures, these higher temperatures approach the melting point of the frit where slumping and excessive grain growth take place. Finally, the firing range between the avoidance of cristobalite and the partial melting of the frit becomes narrower at higher silica (between 74% and 80% SiO.sub.2) with an absolute difference of only about 15.degree.C. at 77% SiO.sub.2 (plus 19.1% Al.sub.2 O.sub.3 and 4.2% Li.sub.2 O). Efficient production of ware with such a narrow firing range would be very costly, and consequently the need for a way of suppressing cristobalite at lower firing temperatures was needed.